In many types of minority carrier semiconductor devices, for example, photodetectors, solar cells, light emitting diodes and laser diodes, device performance may be limited by the surface recombination of carriers. The surface recombination of carriers is generally undesirable and may limit device efficiency for at least two reasons. First, the efficiency of minority carrier devices is reduced because carriers are lost in the recombination process and are, therefore, not collected. Second, surface recombination inevitably leads to some heating of the semiconductor surface, and as the device temperature rises, the device is more likely to fail.
Several techniques have been used to reduce surface recombination of carriers and surface state density. For example, a lattice matched heterojunction formed by growing a layer of Ga.sub.1-x Al.sub.x As on the air-exposed surface of GaAs reduces the recombination velocity and thus the surface recombination of carriers. The surface state density may be altered by well-known surface treatments such as the growth of native oxides or the deposition of SiO.sub.2 or silicon nitride. However, altering of surface state density does not always affect surface recombination of carriers.
Surface treatment of a semiconductor in several GaAs devices has been shown to greatly improve device efficiency. Chemical treatment of the GaAs electrode in a semiconductor-liquid junction solar cell has been shown to increase device efficiency. In Applied Physics Letters, 33, pp. 521-523, Sept. 15, 1978, an efficiency of approximately 12 percent for a semiconductor-liquid junction solar cell having a chemically treated electrode surface was reported. An efficiency of less than 9 percent was obtained for a similar GaAs cell without a chemically treated electrode surface. The particular device described had a fraction of a monolayer of ruthenium on the GaAs electrode surface that contacted an aqueous selenide-polyselenide solution. It was hypothesized in the article that the ruthenium on the GaAs electrode surface altered the surface states initially within the GaAs bandgap in such a way that surface recombination was reduced and device efficiency increased.
A similar effect has been observed with devices using polycrystalline films. For example, Journal of the Electrochemical Society, 127, pp. 90-95, January 1980, described chemically treated polycrystalline gallium arsenide which produced large increases in the efficiency of solar cells upon chemical treatment with acid ruthenium (III) and/or basic lead (II) solutions.
Recently, relatively efficient p-type semiconductor-liquid junction solar cells have been reported. For example, Applied Physics Letters, 38, pp. 282-284, Feb. 15, 1981, reported a semiconductor-liquid junction solar cell using a p-type InP electrode and a V.sup.3+ /V.sup.2+ redox couple in the electrolyte. Cells of this type have achieved a solar to electrical energy conversion efficiency of 11.5 percent. In this cell, V.sup.3+ is reduced to V.sup.2+ at the p-type surface by the photogenerated electrons. V.sup.2+ is oxidized to V.sup.3+ at the counterelectrode by holes. The recombination of electrons and holes at the InP-electrolyte interface is reduced, if not totally prevented, by a thin oxide layer which is formed in situ. The oxide, which is possibly of only monolayer thickness, is strongly bound. It was hypothesized in the article that the strength of chemical bonds at surfaces and grain boundaries is inversely correlated with recombination velocities, and such strong bonding therefore implies reduced carrier recombination.
Although results produced by the in situ oxidation are adequate for many purposes, other methods of treating the surface and improving device characteristics would be desirable. Other methods would be particularly desirable for polycrystalline semiconductor films in which grain boundaries may be insulated from each other upon oxidation.